Electrohydraulic lithotripsy has been typically used for breaking calcified deposits or “stones” in the urinary or biliary track. Recent work by the assignee shows that lithotripsy electrodes may similarly be useful for breaking calcified plaques in the wall of a vascular structure. Shock waves generated by lithotripsy electrodes may be used to controllably fracture a calcified lesion to help prevent sudden stress and injury to the vessel or valve wall when it is dilated using a balloon. A method and system for treating stenotic or calcified vessels is described in U.S. Pat. Nos. 8,956,371, 8,888,788, and 9,011,463 incorporated herein by reference. A method and system for treating stenotic or calcified aortic valves is described in U.S. Pat. No. 9,044,618 incorporated herein by reference. As described in those applications, a balloon is placed adjacent leaflets of a valve or vessel to be treated and is inflated with a liquid. Within the balloon are one or more shock wave electrodes that produce shock waves that propagate through the liquid and impinge upon the valve or vessel. The impinging shock waves soften, break and/or loosen the calcified regions for removal or displacement to open the valve or vessel (e.g., to enlarge the valve opening or clear the lumen of the vessel).
In electrode-based lithotripsy systems, a pair of electrodes is used to generate a shock wave. When a high voltage is applied across the electrode pair, a plasma arc forms between them, giving rise to a steam bubble in the fluid. A first shock wave occurs when the steam bubble first forms and a second shock wave occurs as the steam bubble collapses. The timing and size of the bubble, along with the sonic output and propagation direction of the resultant shock waves, depend at least in part on the location, geometry and size of the electrodes. The size and arrangement of the electrodes also impact the types of vascular structures that may be accessed and treated by the shock wave catheter. For example, coaxial shock wave electrode pairs can comprise an inner wire that is inserted within an outer cable, where the conductive portions of each are exposed at the tips. In this arrangement, a high voltage applied across the inner wire and outer cable gives rise to a shock wave that propagates away from the exposed tips of the inner wire and outer cable. Additional improved lithotripsy or shock wave electrodes that can readily access and generate sufficient sonic output to treat various locations in the vasculature for angioplasty and/or valvuloplasty procedures may be desirable.